The present invention is directed to a novel machine utilizing a modified "Stirling cycle". The first Stirling cycle machine was invented in 1816 by Robert Stirling. It ran as an engine, turning heat into mechanical energy. Subsequent development has shown that Stirling cycle machines can also be run in reverse, being driven by mechanical energy to act as heat pumps in refrigeration applications. Practical problems, discussed below, have prevented Stirling cycle machines from coming into widespread use in any of their potential applications.
Conventional Stirling cycle machines operate with working gas such as air, hydrogen or helium. When the Stirling cycle machine is run as an engine, the working gas is compressed while being cooled in the cold space of the engine during the "compression" phase of the cycle. The working gas is then permitted to expand into the hot space of the engine where it is heated as it expands during the power stroke phase of the cycle. The working gas is then transferred out of the hot space and into the cold space of the engine at constant volume during the "regeneration" phase of the cycle. The cycle then repeats.
The expansion of the working gas in the hot space of the Stirling cycle machine during the power stroke of the cycle produces work when the machine is run as an engine. The compression phase of the cycle absorbs work when the Stirling machine is run as an engine, but it absorbs less work than is generated in the expansion phase of the cycle. The excess work is absorbed in part by mechanical and gas friction (including that involved in the transfer/regeneration phase.) The remainder of the work is useful work.
When run as a heat pump, a Stirling cycle machine requires more energy to compress the working gas during the compression phase of the cycle than is returned during the expansion phase of the cycle because the part of the machine that is absorbing heat from the surroundings (i.e., the expansion space) is colder than the part of the machine where the working gas is compressed (i.e., the compression space).
The present invention is described below as an engine, with reference to the compression space as the "cold space" and the expansion space as the "hot space". If the machine were operated as a refrigerator (heat pump) rather than as an engine, temperatures of the compression and expansion space would be reversed. Both when the invention is used as an engine and as a heat pump, gas entering the expansion space is subjected to external heating, and gas entering the compression space is subjected to external cooling.
Numerous machines embodying variants of the Stirling cycle have been described. (See Walker, Stirling Engines, Claredon Press (1980); Stirling Engine Design and Feasibility for Automotive Use, Collie ed., (Noyes Data Corp. 1979).
The theoretical advantages of the Stirling cycle engines are considerable. They are, in theory, highly efficient. Also, since their heat sources are external to the working gas, some of the air pollution problems associated with internal combustion engines can be avoided. Since fuel for a Stirling cycle engine can be burned steadily at atmospheric pressure rather than exploded at high temperature and pressure, Stirling cycle engines are comparatively quiet. They may be powered by any available source of heat and can thus operate with any type of fuel, including solar heat, geothermal heat or heat from nuclear fission or fusion.
However, a number of practical problems have prevented commercial use of Stirling cycle machines. Those problems include the following:
(1) The mechanical movement necessary to accomplish the cycle (i.e., reduce the volume of the working gas in the cold space, then increase the volume of the working gas in the hot space, then simultaneously reduce the volume of the working gas in the hot space and increase the volume of the working in the cold space) has required complex and expensive arrangements of gears and levers, or the use of multiple cylinders entailing significant weight and friction losses.
(2) Energy loss through friction of the working gas has been a major problem in Stirling cycle machines. Modern Stirling machines heat and cool the working gas by passing it through hot and cold tubes located, respectively, on either side of a regenerator. The use of multiple tubes increases the surface area available for heat transfer relative to a single, large diameter tube, but also greatly increases gas friction. Stirling engines normally employ a "regenerator" of knitted wire, wire mesh, or some similar material between the sets of hot and cold tubes. The regenerator functions by absorbing heat from the gas at one time and returning the heat to the gas at a later time. The regenerator also produces substantial gas friction as the working gas passes back and forth through it, dissipating energy.
(3) The conventional arrangement also creates a thermodynamic problem in that the working gas is passed through the hot heat-exchanger at times during the cycle when it would be desirable for the working gas to be cooled and through the cold heat exchanger at times when it would be desirable for the working gas to be heated.
The present invention accomplishes improvements in all of these areas, producing an efficient thermodynamic cycle with mechanical apparatus employing simple harmonic motion with reduced gas friction.
In one aspect of the invention, improvements in Stirling engine type devices are obtained by employing two separate volumes of gas which are expanded and compressed through Stirling type cycles, sequentially sharing expansion and compression chambers with one volume undergoing expansion while the other undergoes compression.
In another aspect of the invention, gas enters and exits the expansion space of the engine through different ports and a regenerator at the entrance port retains heat of compression and may permit superheating of the working gas in the regenerator.
It is an object of the present invention to provide improved high power, high speed, modified Stirling cycle engines.
It is another object of the present invention to provide improved heat pumps operating in a modified Stirling cycle.
It is a further object of the present invention to provide a low pressure, low speed, modified Stirling cycle engine operative by low quality heat sources such as solar energy.
Other objects will be apparent from the following description.